Conventionally, a shot peening process has been used in various fields, where the shot peening process involves impacting a surface of a workpiece with hard balls of metal, ceramic or the like at high velocities to harden the surface of the metal workpiece. By impacting the workpiece surface with hard balls at high velocities, the shot peening process forms a compressively stressed portion on the workpiece surface with a residual compressive stress imparted thereto, and thereby hardens the workpiece surface and improves its wear resistance.
Patent Literature 1 describes a casting mold whose cooling water passage is subjected to a shot peening process on a surface layer and thereby imparted with a residual compressive stress. In the casting mold, the shot peening process is applied to the surface layer of the cooling water passage to increase fatigue strength against tensile stress produced on the surface layer of the cooling water passage and thereby prevent cracks.